Eutrophication, which is mainly caused by the excessive emission of nutrients such as nitrogen, phosphorus and the like into the environment, is becoming a more and more serious, global environmental problem. As a result, the countries in the world are all upgrading the sewage discharge standard thereof. According to standard A in Level I standard in the current national standard Discharge Standard of Pollutants for Municipal Sewage Treatment Plant (GB 18918-2002), the amount of discharged ammonia nitrogen shall not exceed 5 mg/L (not higher than 8 mg/L at low temperature), total nitrogen (TN) shall not exceed 15 mg/L, and total phosphorus (TP) should not exceed 0.5 mg/L. At present, it is mandatory that the Standard should be strictly enforced when a new sewage treatment facility is established in a river basin or area where water is seriously polluted or higher water quality is required. Currently operated two-stage sewage treatment plants mainly aiming to remove carbonaceous organic materials should be also upgraded and rebuilt gradually, so that the discharged water can meet the requirements stated in the Standard.
Due to relatively low concentration of nitrogen and phosphorus in municipal sewage and huge discharge of sewage, the removal of nitrogen and phosphorus by merely using physical and chemical methods has significant disadvantages of large amount of dosage, large amount of chemical precipitate, and high operation cost, and is seldom adopted in practical sewage treatment projects. The technology of biological nutrient removal (BNR) utilizing a principle of biological nitrogen and phosphorus removal is the major technology adopted in the field of nitrogen and phosphorus removal from municipal sewage at present.
The principle of conventional biological nitrogen removal can be briefed as: (1) firstly, the organic nitrogen, protein nitrogen and the like existing in sewage is converted into ammonia nitrogen by ammonifiers, and then converted into nitrate nitrogen by nitrobacteria under aerobic conditions (this stage is called aerobic nitrification). (2) the nitrate nitrogen is reduced to nitrogen gas by denitrifying bacteria under anoxic conditions using the energy provided by a carbon source, and then escaped from water (this stage is called anoxic denitrification). Nitrification and denitrification are two separate processes involving in two different kinds of microbes. Generally the two processes can not happen at the same time, but can happen sequentially in terms of space or time, since they require different environmental conditions. In a biological nitrogen removal system, nitrobacteria require an aerobic environment, grow in slow speed, and have a sludge age (or biological solids retention time, SRT) of generally no less than 30 days. The denitrifying bacteria generally grow under an anoxic condition, and require sufficient carbon source to provide energy, and the denitrification requires that the ratio of BOD5 (five day biochemical oxygen demand) to TKN (total Kjeldahl nitrogen) is 5-8, otherwise, the denitrification can not happen smoothly unless additional carbon source is provided.
The conventional principle of phosphorus removal can be briefed as: (1) anaerobic stage: soluble organic materials are transformed by the fermentation of facultative bacteria into low molecule weight fermentation product, that is, volatile fatty acids (VFA), which are then absorbed by phosphorus accumulating organisms (PAO), transferred into cells, and assimilated into carbon energy storage material (PHB/PHV) in the cells, by using the energy from poly-P hydrolysis (resulting in the release of phosphorus out of the cells) and the glucolysis in the cells. (2) aerobic stage: under aerobic conditions, phosphorus accumulating bacteria obtain energy from the oxidative metabolism of PHB/PHV for absorbing phosphorus, and synthesizing and storing poly-P in cells, while synthesizing new cells of phosphorus accumulating bacteria and producing phosphorus-enriched sludge. The energy produced in oxidation process is stored as high-energy polyphosphorous ATP. (3) phosphorus removal: phosphorus is finally removed from water treating system by discharging the phosphorus-enriched sludge produced in the aerobic process. Therefore, the conventional biological phosphorus removal process is actually achieved by using phosphorus accumulating bacteria which release phosphorus under anaerobic condition and absorbing excessive phosphorus under aerobic condition, and the only approach thereof to remove phosphorus is to discharge excess sludge. Therefore, the shorter the sludge age is, the better the effect of phosphorus removal is. An ideal sludge age is 3.5-7 d. In addition, the amount of the phosphorus absorbed under aerobic condition is limited by the amount of the phosphorus released under anaerobic condition, while the amount of the phosphorus released under anaerobic condition, on one hand, requires relatively strict anaerobic environment (DO (dissolved oxygen) should be strictly controlled at a value lower than 0.2 mg/L), and on the other hand, is closely related with the concentration of VFAs. Studies showed that, the ratio of BOD5 to TP should be controlled at 20-30, and the content of VFAs in BOD5 should be higher, if the content of phosphorus in the discharged water after treatment is to be controlled at a value lower than 1.0 mg/L.
Since the new discharging standard requires that an municipal sewage treatment plant should achieve highly efficient removal of carbon, nitrogen, and phosphorus at the same time, therefore, the process selected by the municipal sewage treatment plant should have the functions of both nitrogen removal and phosphorus removal at the same time. As discussed above, according to the principle of conventional biological nitrogen and phosphorus removal, the biological treatment process having the function of both nitrogen and phosphorus removal should be able to create environments alternated by anaerobic, anoxia, and aerobic conditions in a certain order for different kinds of microbes. The environments alternated by anaerobic, anoxia, and aerobic conditions in a certain order can be divided by space or time sequence. At present, the sewage treatment processes having the effect of biological nitrogen and phosphorus removal for municipal sewage treatment can be divided into two classes: continuous flow activated sludge method divided by space and batch-type activated sludge method (or sequencing batch reactor) divided by time. The former is represented by anaerobic-anoxia-aerobic (that is, A-A-O or A2/O) process (the technical principle of which is shown in FIG. la), and comprises various modified A2/O processes based on the A2/O processes, for example, A-A2/O process, inverted A2/O process, modified Bardenpho process, UCT process, MUCT process, VIP process, and the like. The latter is represented by sequencing batch reactor (SBR) (the process principle of which is shown in FIG. 2), and comprises various modified SBR processes based on conventional SBR process, for example, ICEAS process, DAT-IAT process, CAST process, CASS process, Unitank process, MSBR process and the like. Continuous flow activated sludge method divided by space is multi-tank biological treatment system, which combines a sludge return system with a mixed liquid return system so that the activated sludge is sequentially subjected to anaerobic, anoxic, aerobic environments or anaerobic, aerobic, anoxic environments in space, so as to achieve and enhance the effect of biological nitrogen and phosphorus removal at the same time. Conventional sequencing batch reactor adopts a single-tank biological treatment system, and has no sludge return system or mixed liquid return system. Therefore, the biochemical reaction and the sludge-water separation by participation are carried out in one reactor. The sewage is fed into the reactor in batches, and treated by the mode of “sewage feeding-reaction-sedimentation-water and sludge discharging-resting” in batches. Anaerobic, aerobic, anoxia environments are sequentially formed in time in the reactor. In order to improve the space utilization of conventional SBR and enhance the ability of nitrogen and phosphorus removal thereof, various modified SBR processes borrow ideas from a multi-tank system, adding a sludge return system or a mixed liquid return system, but at the same time losing some characteristics of conventional SBR process, for example, the characteristics of ideal plug-flow type reactor in time, low probability of sludge bulking, ideal resting and sedimentation, and the like.
At present, the phenomena that nitrogen removal and phosphorus removal can not reach the best effect at the same time when applying a sewage biological nitrogen and phosphorus removal process, especially a multi-tank biological nitrogen and phosphorus removal system, in practice is often observed, that is, good nitrogen removal effect and poor phosphorus removal effect, or good phosphorus removal effect and poor nitrogen removal effect. The reason mainly lies in that the biological nitrogen removal process and the biological phosphorus removal process are contradictory or competitive as follow: (1) biological nitrogen and phosphorous removal comprises the processes of aerobic nitrification, anoxic denitrification, anaerobic phosphorus release, and aerobic phosphorus absorption, and are completed by different kinds of microbes requiring different kinds of substrate and environmental conditions. (2) There is irreconcilable contradiction between the long sludge age as required by the nitrification and the short sludge age as required by biological phosphorus removal. (3) Both anaerobic phosphorus release and anoxic denitrification need a certain amount of carbonic organic materials, especially VFAs. However, since the concentration of VFAs in municipal sewage is generally low (dozens mg/L), the competition caused by insufficient carbon source leads to that phosphorus accumulating bacteria are not dominant. (4) Nitrate imposes adverse influence on the anaerobic phosphorus release. On one hand, nitrate stimulates denitrifying bacteria to compete VFAs with phosphorus accumulating bacteria for biological denitrification, and on the other hand, when the amount of phosphorus accumulated by the phosphorus accumulating bacteria is not high and the level of VFAs in fed water is low, NO3− can induce phosphorus accumulating bacteria to absorb phosphorus under anaerobic condition, and inhibits the process of anaerobic phosphorus release, in turn the effect of biological phosphorus removal.
In improving the existing sewage biological nitrogen and phosphorus removal process, most of the researchers focused on the development of so called enhanced biological phosphorus removal (EBPR), that is, on the way to fully utilize the advantage of phosphorus accumulating bacteria, for example, increasing the number of the reactor, increasing a cycling return pipe to eliminate the inhibition of DO and NO3− carried by the sludge return system to the anaerobic phosphorus release of phosphorus accumulating bacteria. Obviously, such method will lead to not only the increase in investment and operation cost of sewage treatment system, but also the increase in the number of the reactors and the return pipes. In addition, the sludge ratio, retention time and the distribution of substrate load affect the effect of biological nitrogen and phosphorus removal from another aspect.
The biological phosphorus removal method is usually operated in low cost. However, the experiences in operation of a large number of biological phosphorus removal systemes both in China or aboard showed that, it is quite difficult to constantly keep a TP concentration of lower than 1 mg/L in discharged water. For the purpose of overcoming the disadvantages of biological phosphorus removal system, in the water flow direction, that is, the main flow path in the practical sewage treatment project in US, Europe and China, a chemical phosphorus removal tank is finally arranged so as to form a biological-chemical combined phosphorus removal system, serving as a means for ensuring standard-met phosphorus release. However, the main flow path chemical phosphorus removal suffers from significant problems such as high amount of water to be treated, large dosage, high cost of agents, low agent utilization, large amount of precipitated sludge, low phosphorus content, difficulties in handling, inconvenient phosphorus recycling and reuse, and the like, therefore, it is difficult to be carried out in practical sewage treatment processes.
Phostrip side flow phosphorus removal process was developed in 1960s in the field of sewage treatment (the process principle is shown in FIG. 1b). Such process comprises adding an anaerobic phosphorus release tank in the sludge return direction, that is, the side flow path to a conventional activated sludge method (CAS), so that the phosphorus released by the phosphorus releasing tank flows with the supernate into a chemical phosphorus reaction-participation tank, precipitated by lime or other precipitators therein, being subjected to solid-liquid separation in an initial sedimentation tank or a separate flocculation/sedimentation tank, and finally removed from the system as a chemical precipitate. 10%-30% of the amount of the original sewage is split into an anaerobic phosphorus release tank, where the sludge is retained for an average of 5-20 h (hours), and generally 8-12 h. Phostrip side flow phosphorus removal process can keep a final TP concentration of lower than 1 mg/L in the discharged water from the main path of the sewage treatment, substantially free from the influence by the concentration of organic substance in the fed water, and uses significantly lower amount of chemical agents, lowering the cost of the agents, in comparison with the method adopting a chemical phosphorus removal in the main flow path. However, such process has very limited application up to now due to the following disadvantages: the process lacks nitrogen removal function; the phosphorus-enriched supernate in the anaerobic phosphorus release tank can be elutriated in a limited ratio by injecting elutriating water such as the discharged water from the initial sedimentation tank, the discharged water from the secondary sedimentation tank, or the supernate from the lime sedimentation reactor; substantial suspensions are remained in the supernate; the amount of the chemical precipitate is relatively high; and relatively high operation techniques of the process are required for the operator.
Recently, a modified UCT(University of Cape Town) process, that is, BCFS(biologisch chemisch fosfaat stikstof verwijdering) process (the process principle of which is shown in FIG. 1c) is developed in Kluyver biotechnology laboratory in Delft University of Technology in Dutch. Two reactors, that is, a contacting tank and a mixing tank, are separately provided between the anaerobic tank and the anoxia tank of the main path of the UCT process, and between the anoxia tank and the aerobic tank, so that a 3-step process in the conventional biological phosphorus is changed into a 5-step process, 2 cycling systems are changed into 4 cycling systems, that is, the main path of the process is consisted of 5 reactors and 3 groups of internal cycling systems, and the side flow path is consisted of 1 group of sludge cycling system and 1 phosphorus enriched supernate (from the end of the anaerobic tank, with amount being 1/10 of the total amount of the original sewage) chemical sedimentation phosphorus removal unit. The contacting tank is equivalent to a second selector, which can effectively prevent the filamentous sludge bulking caused by the product of anaerobic hydrolysis, and the NO3—N carried in the retuned sludge can be denitrified in the contacting tank to removal phosphorus; the mixing tank is controlled to be operated in low DO (≦0.5 mg/L) state, so that good nitrification and denitrification can be achieved at the same time. On one hand, BCFS process effectively combines aerobic phosphorus absorption, anoxia phosphorus release, and off-line chemical sedimentation of phosphorus-enriched supernate, so that the system has good phosphorus removal effect, with a total phosphorus ≦0.2 mg/L in the discharged water. On the other hand, the process combines the conventional biological nitrogen removal, simultaneous nitrification and denitrification, denitrification phosphorus removal, so as to ensure the system an excellent biological nitrogen removal effect, and a total nitrogen ≦5 mg/L in the discharged water. Therefore, this process is a method that can achieve the best nitrogen and phosphorus removal effect at the same time. However, the BCFS process is extremely complicated, and involves in high infrastructure investment, high operation cost, large occupation, and complicated operation management. Furthermore, the approach for removing phosphorus is mainly achieved by discharging phosphorus-enriched excess sludge, the on-line phosphorus separation and the off-line sedimentation of the anaerobic phosphorus-enriched sewage are only used as an assistant means, and the problems regarding the disposal of large amount of excess sludge and the recycling of the phosphorus resources from the sewage remain unresolved. Therefore, the application of BCFS process in practical sewage treatment project is significantly limited.
In view of the problems regarding a poor nitrogen removal effect of Phostrip side flow phosphorus removal process and the complicated BCFS process, a new biological nitrogen and phosphorus removal process using “external recycle process of aerobic sludge in SBR system” (ERP-SBR) was proposed by Chongqing University (the process principle of which is shown in FIG. 3). The ERP-SBR system additionally provides an enhancing anaerobic phosphorus release tank (that is, an anaerobic reactor) and a chemical phosphorus removal tank to the conventional SBR reactor. Rather than achieving an on-line separation of the phosphorus-enriched supernate at the end of the anaerobic tank of the main flow path in BCFS process, the SBR system performs sedimentation and water discharging, and discharges part of the aerobic phosphorus absorbing sludge together with part of the original sewage to an anaerobic phosphorus release tank, where the sludge subjected to anaerobic phosphorus release is rested and precipitated for sludge-water separation, and then recycled into the SBR reactor and involved in the aerobic phosphorus absorbing process again. The phosphorus-enriched supernate is discharged into a chemical phosphorus removal tank, reacted with the added chemical phosphorus removing agent to produce phosphorus-enriched precipitate, precipitated, and recycled into the SBR reactor for further removing pollutants such as ammonia, phosphorus, organic substance and the like in the sewage. The ERP-SBR process is greatly simplified over BCFS process, and basically replaced the way by which the conventional biological phosphorus removal process discharges aerobic phosphorus-enriched sludge in the main path with a side-flow discharging anaerobic phosphorus-enriched sludge to remove phosphorus. Therefore, the sludge age, the concentration of the sludge, and the activity of the sludge in the SBR reactor are improved, the resistance to shock load is enhanced, the competition of denitrification and anaerobic phosphorus release for carbonic organic substance in conventional nitrogen and phosphorus removal system is avoided, and good effect of nitrogen and phosphorus removal is achieved. Furthermore, by chemically treating phosphorus-enriched sewage comprising only about 1/10 of the total amount of the original sewage but having a concentration of the phosphorus about ten times that of the original sewage via off-line enhanced anaerobic phosphorus release, the size of the chemical phosphorus removal system is reduced, and the dosage of the chemical agents and the yield of the chemical precipitate are decreased. The agent utilization in chemical phosphorus removal system and the phosphorus content in the chemical precipitate are improved, which in turn favors the recycling and utilization of the phosphorus resource in the phosphorus-enriched chemical precipitate, and eliminates the stage of fixing and disposing the phosphorus in the phosphorus-enriched sludge discharged by the conventional biological phosphorus removal system.
However, ERP-SBR process still has the following disadvantages: (1) the controlling of the sludge age is in dilemma: shortening sludge age is not advantageous for the reason of disfavoring biological nitrogen removal and increasing the disposal cost of the sludge, while prolonging the sludge age can maximally recover and reuse the phosphorus in the sewage as phosphorus-enriched sedimentation; however, prolonging the sludge age will gradually increase the concentration of the sludge in SBR reactor serving as the main reacting tank as the run time is lengthened, the efficiency of sludge-water separation due to gravity sedimentation is gradually decreased, the surface of the sludge is lifted, and the volumetric exchange ratio of the SBR reactor is reduced, which further decrease the space utilization of the SBR reactor, and even cause sludge bulking and degradation in water quality of the discharged water in serious situation; (2) using a decanter as the water decanting device for decanting the supernate at the end of sedimentation in the SBR reactor as the final discharged water, the SS (solid suspension) in the discharged water is difficult to be decreased below 10 mg/L; and the TP carried in the SS in the discharged water is usually 0.5 mg/L since the phosphorus content of the SS in the supernate at the end of the sedimentation in the SBR reactor is generally no less than 5%, so that the discharge of the whole system can hardly meet the requirements about TP as stated in Level A standard of GB18918; (3) the process of anaerobic phosphorus release is short itself, generally about 2 h, however, in the case of high sludge concentration, the sludge-water separation by means of gravity sedimentation in the anaerobic phosphorus release tank needs long precipitating time, and lifts the sludge surface, so that the discharging height of the phosphorus-enriched supernate that can be discharged into the chemical phosphorus removal tank is greatly reduced. Even if the anaerobic phosphorus release has been sufficiently finished, the phosphorus-enriched supernate that can be efficiently discharged into the chemical phosphorus removal tank is less than half of the total phosphorus-enriched sewage, so that the phosphorus removal ability of the system can not be further improved.